We propose to study in vitro the transcription of UV-irradiated DNA and chromatin templates of E. coli RNA polymerase and by eukaryotic RNA polymerase II. The purpose of these studies is to: 1) provide a high resolution analysis of the photochemical and biochemical events which are responsible for the premature transcription termination on UV-irradiated template DNA: 2) provide a better understanding of the effects of UV photoproducts on the function of control regions; 3) provide information regarding the template properties of UV-irradiated chromatin; and 4) work towards establishing a firm molecular basis of UV transcription mapping eukaryotic chromatin/RNA polymerase II system. UV mapping of transcription units has become an important technique for in vivo determinations of transcriptional distances. The technique has advantages over some of the other methods for sizing transcription units in that it does not depend on the isolation of full size primary transcripts and it does not suffer from a rapid turnover of portions of primary transcripts. Important details of the photochemical principles which form the basis of the UV mapping technique are still unknown. The proposed work will provide these details and will increase the utility of the method especially in applications to transcriptional analyses in eukaryotic cells.